The present invention pertains to the radio frequency art and, more particularly, to a means for coherently combining a plurality of radio frequency signals.
Lenses for focusing radio frequency signals are well known, especially in the radio communication art. There, for example, multi-element antennas have been used for the reception of remotely broadcast radio signals. Due to the spatial separation among the antenna elements, a wavefront impinging on the antenna array strikes some antennas sooner than others, thereby producing phase differences. If all antenna element signals are combined without phase correction, the resulting signal would be incoherent, or out of focus.
One approach to providing phase coherency is that of the Rotmen lens. Here, each antenna element is coupled through a predetermined coaxial cable to a focal point. The lengths of the cables compensate for the spatial positioning of the antenna elements such that all signals are coherent at the focal point. Thus, a separate set of coaxial cables are required for each desired focal point, thereby resulting in an expensive and bulky lens assembly. Further, a Rotman lens has established limitations in its overall coverage angle.
A more flexible prior art lens than those of the Rotman type is an R-KR lens. In such a lens, which is illustrated in FIG. 1, and which is more fully described herein below, input and output probes are located in predetermined locations in a cylindrical transmission cavity. The dimensions of the cavity and the positioning of the probe sites are such that signals induced in an antenna element from a given signal front are sent to appropriate probes whereby, due to the varying distance between antenna signal probes and a focal probe the desired phase corrections are made thus resulting in a focused image at the focal probe. A major problem with systems of this type is that due to the radiation pattern of each probe, internal reflections within the cavity may be generated thus resulting in phase incoherencies and an unfocused image. One approach to minimizing reflections has been to fill the cavity with a lossy dielectric material, thereby reducing the amplitude of reflected signals. This approach is undesirable due to an increased system loss of between 3 and 10db. Another attempt to minimize phase incoherencies has been to provide two probes at each probe site, with the two probes being driven in phase. Though this approach was superior to the single probe case, substantial defocusing of the lens still occured.
A further prior art approach is the Luneberg lens. Here, a transmission cavity is comprised of two circular plates which form a transverse electromagnetic wave (TEM) cavity. Probes are placed within the cavity as in the R-KR lens. The region between the plates is partially filled with a tapered dielectric material. While this system is effective in reducing some incoherent signal images, it is costly to build.